Platinum catalyzed fluorosilicone/organosilicone blends and low molecular weight silicone copolymer compositions such as those described in U.S. Pat. No. 4,122,247 (Evans), U.S. Pat. No. 4,157,337 (Evans), and U.S. Pat. No. 4,317,899 (Bluestein et al) have found use as low energy molding compounds in the aerospace industry for making high quality electrical connectors and other molded silicone rubber inserts. Such compounds, which cure via hydrosilation addition reaction in the presence of platinum, display the advantages of rapid processing, fast deep section curing, low rejection rate, and good mechanical properties. However, in spite of these advantages, the addition cure systems are susceptible to catalyst poisoning, e.g., from small amounts of compounds such as carbon monoxide or sulfur or amines, which tie up platinum; the ratio of silicon hydride to olefin functionality for optimal properties must fall within a narrow range; a critical range of inhibitor to catalyst is necessary to provide rapid cure along with adequate working shelf life; and the systems are also susceptible to organic contaminants which can adversely affect the thermal stability of the cured parts.
In order to secure better thermal aging properties, many electrical connector manufacturers have tested peroxide-cured fluorosilicone systems made by blending heat-curable fluorosilicone compounds with polydimethylsiloxane gums. Because of the nature of the curing reaction, addition cure compositions do not fully cure initially, and further curing in response to high temperatures encountered by the compositions in use may lead to shrinkage of the molded component, making it unsuitable for continued used. Greater dimensional stability is provided by peroxide-curable systems which fully cure and so do not shrink in use. It has been observed that the peroxide-curable blends, however, require a higher overall average fluorine content to provide sufficient solvent resistance. Consequently, there has been a need for a peroxide-curable fluorosilicone copolymer gum which exhibits the desirable processability of the addition cure systems while providing better thermal aging and good resistance to solvent swell.